Gases, Liquefaction of

Liquefaction of gases is the process by which substances in their gaseous state are converted to the liquid state. When pressure on a gas is increased, its molecules closer together, and its temperature is reduced, which removes enough energy to make it change from the gaseous to the liquid state.

Two important properties of gases are important in developing methods for their liquefaction: critical temperature and critical pressure. The critical temperature of a gas is the temperature at or above which no amount of pressure, however great, will cause the gas to liquefy. The minimum pressure required to liquefy the gas at the critical temperature is called the critical pressure.

For example, the critical temperature for carbon dioxide is 304K (87.8°F [31°C]). That means that no amount of pressure applied to a sample of carbon dioxide gas at or above 304K (87.8°F [31°C]) will cause the gas to liquefy. At or below that temperature, however, the gas can be liquefied provided sufficient pressure is applied. The corresponding critical pressure for carbon dioxide at 304K (87.8°F [31°C]) is 72.9 atmospheres. In other words, the application of a pressure of 72.9 atmospheres of pressure on a sample of carbon dioxide gas at 304K (87.8°F [31°C]) will cause the gas to liquefy.

Differences in critical temperatures among gases means that some gases are easier to liquefy than others. The critical temperature of carbon dioxide is high enough so that it can be liquefied relatively easily at or near room temperature. By comparison, the critical temperature of nitrogen gas is 126K (–232.6°F [–147°C]) and that of helium is 5.3K (–449.9°F [–267.7°C]). Liquefying gases such as nitrogen and helium obviously present much greater difficulties than does the liquefaction of carbon dioxide.

In general, gases can be liquefied by one of three methods: (1) by compressing the gas at temperatures less than its critical temperature; (2) by making the gas do some kind of work against an external force, which causes the gas to lose energy and change to the liquid state; and (3) by making gas do work against its own internal forces, also causing it to lose energy and liquefy.

In the first approach, the application of pressure alone is sufficient to cause a gas to change to a liquid. For example, ammonia has a critical temperature of 406K (271.4°F [133°C]). This temperature is well above room temperature, so it is relatively simple to convert ammonia gas to the liquid state simply by applying sufficient pressure. At its critical temperature, that pressure is 112.5 atmospheres, although the cooler the gas is to begin with, the less pressure is needed to make it condense.

A simple example of the second method for liquefying gases is the steam engine. The principle on which a steam engine operates is that water is boiled and the steam produced is introduced into a cylinder. Inside the cylinder, the steam pushes on a piston, which drives some kind of machinery. As the steam pushes against the piston, it loses energy. That loss of energy is reflected in a lowering of the temperature of the steam. The lowered temperature may be sufficient to cause the steam to change back to water.

In practice, the liquefaction of a gas by this method takes place in two steps. First, the gas is cooled, and then it is forced to do work against some external system. For example, it might be driven through a small turbine, where it causes a set of blades to rotate. The energy loss resulting from driving the turbine may then be sufficient to cause the gas to change to a liquid.

The process described so far is similar to the principle on which refrigeration systems work. The coolant in a refrigerator is first converted from a gas to a liquid by one of the methods described above. It then absorbs heat from the refrigerator box, changing back into a gas in the process. The difference between liquefaction and refrigeration, however, is that in the former process, the liquefied gas is constantly removed from the system for use in some other process, while in the latter process, the liquefied gas is constantly recycled within the refrigeration system.

In some ways, the simplest method for liquefying a gas is simply to take advantage of the forces that operate between its own molecules. This can be done by forcing the gas to pass through a small nozzle, or a porous plug. The change that takes place in the gas during this process depends on its original temperature. If that temperature is less than some fixed value, known as the inversion temperature, then the gas will always be cooled as it passes through the nozzle or plug.

In some cases, the cooling that occurs during this process may not be sufficient to cause liquefaction of the gas. However, the process can be repeated more than once. Each time, more energy is removed from the gas, its temperature falls further, and, eventually, it changes to a liquid. This kind of cascade effect can, in fact, be used with either of the last two methods of gas liquefaction.

The most important advantage of liquefying gases is that they can then be stored and transported in much more compact form than in the gaseous state. Two kinds of liquefied gases are widely used commercially for this reason, liquefied natural gas (LNG) and liquefied petroleum gas (LPG). LPG is a mixture of gases obtained from natural gas or petroleum that has been converted to the liquid state. The mixture is stored in strong containers that can withstand very high pressures. LPG is used as a fuel in motor homes, boats, and homes that do not have access to other forms of fuel.

Liquefied natural gas is similar to LPG, except that it has had almost everything except methane removed. LNG and LPG have many similar uses.

In principle, any gas can be liquefied, so their compactness and ease of transportation has made them popular for a number of other applications. For example, liquid oxygen and liquid hydrogen are used in rocket engines. Liquid oxygen and liquid acetylene can be used in welding operations. And a combination of liquid oxygen and liquid nitrogen can be used in aqualung devices.

Liquefaction of gases is also important in the field of research known as cryogenics. Liquid helium is widely used for the study of behavior of matter at temperatures close to absolute zero, 0K (— 459°F [— 273°C]).

Pioneer work on the liquefaction of gases was carried out by the English scientist Michael Faraday (1791-1867) in the early 1820s. Faraday was able to liquefy gases with high critical temperatures such as chlorine, hydrogen sulfide, hydrogen bromide, and carbon dioxide by the application of pressure alone. It was not until a half-century later, however, that researchers found ways to liquefy gases with lower critical temperatures, such as oxygen, nitrogen, and carbon monoxide. The French physicist Louis-Paul Cailletet (1832-1913) and the Swiss chemist Raoul-Pierre Pictet (1846-1929) developed devices using the nozzle and porous plug method for liquefying these gases. It was not until the end of the nineteenth century that the two gases with the lowest critical temperatures, hydrogen (–399.5°F [–239.7°C; 33.3K]) and helium (–449.9°F [–267.7°C; 5.3K]) were liquefied by the work of the Scottish scientist James Dewar(1842-1923) and the Dutch physicist Heike Kamerlingh Onnes (1853-1926), respectively.

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